This invention relates to a high strength flooring composition. More specifically, it relates to a flooring composition made using beta-calcined calcium sulfate hemihydrate.
Both gypsum and cement are well known as construction materials. Gypsum is the principal component of the familiar wallboard, where is it faced with paper to provide strength and a smooth surface. Cement is used in various applications where its water resistance and hardness are important, such as in concrete structures. Cement is also used in building panels where its hardness and water resistance are important.
Gypsum is also known as calcium sulfate dihydrate, terra alba or landplaster. Plaster of Paris is also known as calcined gypsum, stucco, calcium sulfate semihydrate, calcium sulfate half-hydrate or calcium sulfate hemihydrate. Synthetic gypsum, which is a byproduct of flue gas desulfurization processes from power plants, may also be used. When it is mined, raw gypsum is found in the dihydrate form. In this form, there are approximately two water molecules of water associated with each molecule of calcium sulfate. In order to produce the hemihydrate form, the gypsum can be calcined to drive off some of the water of hydration by the following equation:CaSO4.2H2O→CaSO4.1/2H2O+3/2H2O
When mixed with water, the hemihydrate rehydrates to form an interlocking matrix of interlocking dihydrate crystals. Gypsum hydration occurs in a matter of minutes or hours compared to several days for cement. This makes gypsum an attractive alternative for cement for flooring applications if sufficient hardness and strength can be achieved in the gypsum.
Calcium sulfate hemihydrate can produce at least two crystal forms. Alpha-calcined gypsum is made by a continuous process or lump rock process whereby the calcium sulfate dihydrate is calcined under pressure. The alpha-calcined gypsum forms less acicular crystals than beta-calcined gypsum, allowing the crystals to pack tightly together, making a denser and stronger plaster. The crystal morphology allows water to flow easily between the crystals, requiring less water to form a flowable slurry. More elongated crystals are characteristic of the beta-calcined gypsum. This crystal structure results in a less dense product because the crystals are more loosely packed. The beta form also requires more water to fluidize the calcined gypsum. In applications where hardness is important, alpha-calcined gypsum is usually preferred, despite higher cost and limited availability.
When choosing a calcined gypsum for an application, beta-calcined gypsum is often selected due to its ready availability and its reduced cost. Because beta-calcined gypsum is also more common, it can incur reduced shipping and storage costs than the alpha form. However, the crystal structure makes it difficult to make a strong, dense gypsum because more water is needed to produce a slurry of a given fluidity.
Superplasticizer additives are known to improve the fluidity of a gypsum slurry. They disperse the molecules in solution so that they move more easily relative to each other, thereby improving the flowability of the entire slurry. Sulfonated melamines and sulfonated naphthalenes are known as superplasticizers, however, the improved fluidity obtained is not sufficient to permit complete substitution of beta-calcined hemihydrate for alpha-calcined hemihydrate in high strength flooring applications.